1. Field of the Invention
The invention relates to a coarse-approach positioning device intended for approaching a sample towards a tool or measuring device, or vice versa. In particular, this device may find application in the microscope art, for example in connection with scanning tunneling microscopes or scanning electron microscopes.
2. Description Of The Prior Art
In the microscope art, it is always necessary to position the sample to be investigated as closely as possible to the point of best observation. In the case of the tunneling microscope the sample should be placed at a distance between 0,1 and 1 nanometer from the tunneling tip, in the case of the scanning electron microscope, the sample should be placed at the focal point of the electron beam, again with a tolerance in the nanometer range. The positioning of a sample to within one nanometer from an optimum location usually cannot be accomplished in a single step. Typically, first a coarse approach is performed by which the sample is brought to within one micrometer of the optimum location, and then a fine approach mechanism takes over to accomplish the positioning to the desired tolerance.
Well known in the microscope art are rack-and-pinion devices for coarse adjustment, and screw appliances for fine adjustment. However, it is to be noted that the terms "coarse" and "fine" when used in connection with optical microscopes define an entirely different magnitude of adjustment than is required in connection, for example, with the tunneling microscope. The reason is that the tunneling microscope has a resolution at least three orders of magnitude better than conventional optical microscopes. In other words, the conventional fine-adjustment appliance of an optical microscope is certainly too coarse to serve as the coarse-approach adjustment mechanism for a tunneling microscope.
Several coarse-approach positioning devices for use with the scanning tunneling microscope have already been proposed. All use piezoelectric elements to accomplish movements in steps of several micrometers each, such as those disclosed in EP-A No. 71 666, EP-A No. 160 707, U.S. Pat. No. 4,520,570, IBM Technical Disclosure Bulletin Vol.26, No. 10A (1984) pp. 4898 and 5175. Coarse and fine adjustment can be performed by a device taught by IBM Technical Disclosure Bulletin Vol. 22, No. 7 (1979) p. 2897.
The devices known in the prior art have one or both of the following disadvantages: They do not operate at cryogenic temperatures or in ultra-high vacuum; they transmit vibrations to the sample so that a sharp image of the sample surface cannot be obtained.